Disposal of radioactive cations



Patented Nov. d, 1952 DISPOSAL OF RADIOACTIVE CATION S William S. Ginell, Levittown, N. Y., assignor tothe United States of America as represented by the United States Atomic Energy Commission No Drawing. Application April.- 27, 1951, Serial No. 223,451

12 Claims.

This invention relates to the disposal of radioactive waste products, especially fission products and in particular the invention relates to an ionexchange process using certain clay minerals for concentrating and storing radioactive Wastes.

One of the important problems tending to impede the use of radioactive materials in large quantities is the disposal of the waste products. This is especially important in the operation of neutronic reactors Where large quantities of radioactive isotopes are formed in the fission of nuclear fuels such as uranium-235 and plutonium. Various methods are now employed for concentrating these wastes, such as evaporation, ion exchange and similar processes. However, even after concentration, there remain large volumes of solutions which must be stored for long periods until the long-lived radioactive isotopes decay to a level safe enough for disposal. The storage of these waste products is very expensive and there is the ever present danger that the solutions will leak from the storage containers and contaminate the water supply systems.

Accordingly, the principal object of this invention is to provide a method for the disposal of waste radioactive metals, especially fission products. Another object of the invention is to provide a method for concentrating and storing fission product wastes. A more specific object of the invention is to provide a method utilizing the ion-exchange capacities of certain clay minerals for concentrating and storing radioactive waste products. A particular object is to provide a method for disposing of the long-lived fission product isotopes of cesium, cerium and strontium. Other objects will become apparent from the following description.

The present invention employs the cation exchange capacity of the montmorillonite minerals. The mineral is brought into contact with the waste solution of radioactive cations to cause the cations to exchange with cations in the mineral. Thereafter the mineral is heated to a temperature greater than 750 C. and up to about 110i) C. to alter its crystal structure. The heat-treated clay is then buried for disposal, preferably in a bed of natural clay. localities as well where contact with ground water is unlikely, for example in abandoned mines or'in concrete chambers.

It may be buried in other One of the properties of the montmorillonite minerals is their ability to adsorb large quantities of water. This process of adsorption is reversible, and the reversibility is attributed to the peculiar crystal structure of the mineral. It is believed, from studies of the structure of the mineral, that the lattice'consi-sts of sheets ofcrystal units. The distance between these sheets perpendicular to the so-called cleavage planes depends upon the quantity of water'which the clay holds. That is, this distance between the crystal layers maybe increased ordecreased reversibly by wetting or drying the mineral respectively. The phenomenon is exhibited by the property of swelling. in contact with Water. It has also been found that upon heating thesemineralsto temperatures of around 1000 C. in order to dehydrate them completely, the mineral loses this property of swelling. It is to be noted that the term drying or dried is used herein in the sense of removing sorbed, i. e. physically bound, water only andat a temperature of less than 300 C.

Another of the properties of the montmorillonite minerals is their ability to exchange cations with a solution, and to some extent also to adsorb various materials. The sum of the two processesis generally referred to as adsorption and no distinction ismade herein between these twoprocesses. The cation-exchange capacities of some of these minerals are listed in Table However, I have discovered that. by heating a montmorillonitemineral to a temperature in the range of 750' to 1100 C. after it has been exposed" to a cation-exchange process, substantially none oi'the' exchanged ions may be extracted from the mineral even underthe most severe conditions of'elution. If a-temperature less than 750 C. is used, the exchanged cations may be relatively easily'remove'd by leaching. In other words; I have found that the normally reversiblezcations exchange process: which takes place between montmorillonite minerals and solutions of cations may be made irreversible by the relatively simple step of heating the mineral containing the adsorbed ions to a temperature in the range of 750 to 1100 C. Thereafter, the exchanged ions on the mineral are substantially impervious to leaching or elution.

Accordingly, my process involves bringing into contact a cation-exchange mineral of the montmorillom'te group and a solution containing radioactive cations. The mineral, separated from the solution, is then heat treated at a temperature in the range 750 to 1100 C. to fix the adsorbed radioactive cations. To dispose of the clay made radioactive in the exchange process, it is preferably deposited in a bed surrounded by ordinary clay. An isolated natural clay bed may be employed for this purpose, for example a bed of kaolin or other ceramic clay. However, similar results can be obtained by burying the heattreated clay in a locality where rainfall is infrequent and vegetation is scarce and surrounding the heat-treated clay with ordinary clay several feet in thickness. Thereby, advantage is taken of the fact that ordinary, naturally occurring clay is Wet and plastic and generally impervious to the flow of water. The clay container limits still further the effects of natural leaching processes. Thus, while the heat treatment of the clay substantially prevents leaching under the most stringent laboratory conditions,

the burial of the heat-treated radioactive clay in this manner provides the further assurance that even after very prolonged exposure of the clay, there will be no elution of the radioactive ions by natural leaching processes. It is, of course,. impossible in the laboratory to simulate with complete accuracy the effect of natural leaching processes over very long periods of time commensurate with the half-lives of some of the radioactive fission products. 7

The following examples will illustrate the effectiveness of my invention for providing a method for the permanent disposal of radioactive cations. It should be understood that it is not intended to limit the scope of this inven tion by the details disclosed in these examples.

EXAMPLE 1 vated) and dried at a temperature of just under 300 F. The mineral as received was heated at 110 C. to constant weight to remove excess water and provide a uniform basis for weighing samples of the mineral. 10 grams of the dry powdery clay (particle size less than 200 mesh) was then suspended in 225 cubic centimeters of the aqueous strontium chloride containing the strontium89 as tracer. The suspension of clay in strontium chloride solution was vigorously shaken until there was no change in the concentration of strontium as determined by measuring the specific activity of the solution. Thereafter the suspended clay was filtered from the solution and washed with distilled water until thewash water revealed no trace of strontium. The filtered clay was then heated to ,constan t weight at 110? C. to

' specific activity.

dry it. A number of samples of the prepared clay were then heated to temperatures of between 300 and 880 C. for a period of time of 24 to 48 hours and then cooled to room temperature.

Aqueous barium chloride solutions were employed to provide an accelerated determination of the effect of natural leaching or elution by ground waters containing dissolved minerals. After the heat treatment, 0.2 gram of each clay sample was suspended in 50 cubic centimeters of 0.5 molar aqueous barium chloride and the suspensions were vigorously and continuously shaken. Very small aliquots were removed from time to time and their specific activities were determined. Barium chloride was used to determine the amount of strontium which could be leached from the clay since barium is known to replace strontium in the clay relatively easily by ion exchange. Since the barium chloride was not radioactive, the concentration of strontium in the solution was determined by measuring the These results are shown in Table I.

Table I Elation Time (days) Percent In 183 days, leaching with distilled water eluted 16.8% of the strontium adsorbed by the sample which had received no heat treatment after drying at C. This clay has an exchange capacity for strontium of 0.462 milliequivalents of strontium per gram of dry clay. It is surprising that after almost six months of continuous leaching with a large excess of vigorously agitated 0.5M barium chloride solution, far more stringent conditions than any natural leaching action, no detectable amount of the adsorbed strontium had been removed from the clay which had been heat-treated at 880 C. Only 1% of the adsorbed strontium was removed from the clay sample heated at 785 C.

EXAMPLE 2 In another experiment the procedure was identical to that described in Example 1 except that the clay used was a relatively pure montmoril lonite, lot number R-2532, which is understood to have been dried by the manufacturer, but not acid-activated. The particle size of the clay was less than 200 mesh. The results of the experiment are shown in Table II.

In 196 days, leaching with distilled water under the same conditions removed 6.7% of the strontium adsorbed by a sample of the clay which had received no heat treatment after dryclay is 02734 milliequivzilents :of strontium :per gram of dry 'elay. Hereagainit is notabl'e that no detectable quantity of strontium was -leached in. more thansix'monthsifrom theclay which had beenrheated 1t0i8'75" '0.

EXAMPLE 3 A solution 'of mixed fission products was .Obtainedby dissolvinginaqueous nitric acid 'ajportion 'ofa uranium rod'which had "been" irradiated in-aneutronic reactor for .a period of. about one year 'and' then retained in storage for 'aboutffour years'to permit'the radioactivity to'd'ec'rease to a relativelysafe handling level. 'FIOm"thlS'SO1l1- tion the major partofthe uranium wasxremoved by successive extractions with 'diethyl ether, a selective solvent for uranium. Theremaining aqueous solution was evaporated vahnostto "dryness to remove ether and EXCBSSfilitIiC acid. The substantially dried residue was dissolvedin distilled water and it was found'that the concentration ofuranyl ion was .1ess;than Iomolar. The "solution had a pH .of "3.5 and its specific activity was 10.58 microcuries'per' milliliter. The fission product analysis of "this solution calculated from theirradiation time and 'coolingt'ime of the uranium rodis'shown in Table III. The miscellaneous contribution to the activity of'the solution includes zirconium-95, niobium 95 and europium-155.

Table .III

flercentaze I v Isotope of Total Half Life 'Curies Cerium-144 25 275 days. Praseodymium144 25 17.51Ulfl1ltt5. Prometheum147 l8 4 years. Strontium-90 A a 8 20 years. Yttrium-90 '8 -'65 hours. Cesium-137 8 33 years. Ruthenium-10G 3 .l year. Rhodium-103 3 30 seconds. Miscellaneous 2 This solution was then treated with the same clay, montmorillonite, under the same conditions as .those employed in Example 1 except that 5.grams of clay were treated with 180 cubic centimeters of the fission product'solu'tion. The samples were afterwards heated to temperatures in the range 455 to 915 C. Thereafter the clays were leached with synthetic .solution whose.composition was approximately the same as sea water from'the Atlantic Ocean. Leaching experiments were also carried out with distilled The fission product solution prepared as .de scribed in Example 3 was usedin an=experiment with the clay employed in Example 2. The procedure of Example 3 was followed except that in this case the heat treatment temperatures 'L'Ihe 'lresults are .Table V Percent 'Heat Treatment E1ution ai fig H Removed Temperature .0.) I ,Time (days) Sea Water plgltlgd Elution time of 63 days.

EXAMPLE 5 The procedure usedin Example 3 was followed using Attapulgus fullers earth designated by the manufacturer 'aslow volatile material. The particle size'o'f this clay was'40 to *'mesh. "The results-are shown in Table W.

Table VI 7 1 i Percent I-IeatTreatnz enCt) I TEluEiion 23221 2 Ilt)einovedd Temperature in'ie' ays 'istille v Sea Water .Water -66 5.0 1 a-o 419 y 3.9 64 "2.5 1 2L 2 52: 1. .2 0.4 '61 "0:54 o. 4

.It will .be noted .that after heat itreatment at 915 C. .only .about .'0';54'% of the adsorbed ions were .removed by the vigorous leaching. Examples..3, 4.andi5 illustrate .the efficacy of the-step of heating a montmorilloniteclay containing adsorbed fission product ions to a temperature of more than 750 C. to bindthe ions intheclay.

EXANIPLE -6 In a similarexperimenta solution 0:017 -molar in inactive cerium nitrate as carrier and containing also the radioactive isotope cerium- 144 was used with the montmorilloniteclay-employed in Example 1. 5 grams of clay were treated with cc. of solution, and the clay capacity was found to be 0.605'mil1iequi-valents if-cerium per gram. The results shownin Table VII 'were-obtained. The procedure or Example 3 was ifollowed.

Tdble 'VH gnu m For cut 0 Heat Treatment Elution Removed Temperature C.) Time (days) I sea a Distilled Water Both cerium :and strontium are of particular importance in the disposal of fission products since "the isotopes of these elements produced by' the fiss'ion process have long half-lives. Examp'les 1, 2 and 6 demonstrate that the heat treatment step of my method is especially *e'ifective for both cerium and strontium.

In order to determine the efiect 'of'the carrier ions :on the adsorption :and subsequent 'heattreatment, 2. parallel experiment was performed :using a solution prepared :by adding .the same @quantity of radioactive cerium-144 to water instead of :to

0.017 molar cerous nitrate. The results obtained are indicated in Table VIII.

Similar results were obtained by following the procedure of Example 6 with a solution 0.0039 molar in cesium chloride containing radioactive cesium-134 as tracer.- After heat treatment of the clay of 24 hours at 900 C., substantially none of the cesium was released on leaching with the synthetic sea water.

In the foregoing examples the leachants used and the methods used for leaching were employed primarily to provide an accelerated test of natural elution by ground water. Similar results were obtained even when 6N nitric acid was employed as eluant. When the heat-treated clays are buried, as described above, they would be subjected to substantially no elution whatever because there is substantially no flow of water through natural clay. Furthermore, by locating the depositories for the heat-treated clays containing the fission products in a relatively arid region the opportunity for leaching by ground, waters is lessened still further.

After heat treatment, the clay containing the bound radioactive ions may be advantageously employed as a source of radiation particularly in treating materials in solution and in medical applications. The heat-treated clay is thermally and chemically quite stable and neither affects nor is affected by its environment.

The clay minerals which are useful in this method are those of the montmorillonite group, i. e. those having as their principal ingredient the mineral montmorillonite, particularly montmorillonite itself, bentonite and fullers earth. The montmorillonite group are the only clays which are known to contain an expanding lattice whose spacing in at least one direction depends upon the quantity of water it contains and my invention is limited to this group. It is my belief that the exchanged ions are located within this lattice which is so altered by the heat treatment that the ions cannot be reached by the eluant.

In addition to the many natural forms of this group of clays containing other substituent minerals, they are commercially available in still other forms after various proprietary treatments. The usual purpose of these treatments is to increase the ion-exchange capacity and the methods employed often vary with the supplier and. with the specific mineral. Accordingly, I have designated the general class as a cation-exchange mineral containing montmorillonite as its principal ingredient. Many varieties of this group of clays are described in the Geological Survey Paper referred to above.

In carrying out the method, it is preferable that the pH of the solution be maintained in the range 4 to 9. When the solution is more acid or more basic than this, there appears to be a decrease in the amount adsorbed so that a larger 8 volume of clay is required per volume of solution processed. The mineral acids as a class are preferred as anions. The time of contact and the ratio of clay to solution treated may easily be found by simple experimental procedures well known in the art.

The temperature of heat treatment of the clay is required to be in the range of 750 to 1100 C. As illustrated in the examples, below 750 0. there is insufficient binding of the adsorbed ions to the clay to insure substantially complete freedom from leaching. At temperatures in excess of 1100 C. there is a tendency to vaporize some of the fission product metals from the clay as silicates. The heating period depends to some extent on the nature of the clay and should be contlnued until sintering is noticed; a, heating time of one-half to two days has been found efficacious depending on the particular mineral but the particle size of the clay has considerable influence on the time required.

The particle size of the clay is quite important. In general the finest possible particles are to be preferred because of the rate of ion exchange and the capacity for ion exchange are both thereby increased. Similarly, with fine particles the heat treatment may be carried out more rapidly. However, with very fine particles the rate of elution is also increased. To compromise between these conflicting influences, it is preferred to employ clay with a particle size not smaller than 1 micron and not larger than 50 microns.

In general, ion exchange processes are more efiiciently performed in a continuous manner. Because it is difiicult to flow a solution through a bed of finely divided clay, it is desirable to support the clay on a suitable carrier such as asbestos, kieselguhr or diatomaceous earth, which permits flow. It has been found that these carriers do not interfere to a measurable extent with the ion-exchange process nor do these carriers exchange or adsorb any appreciable quantity of the radioactive ions.

It is notable that these clays adsorb cations from virtually every group of the periodic table. Thus, cesium is representative of the group I metals and strontium of the group II metals; praseodymium, cerium, promethium and yttrium are representative of the group III metals and the rare earths while ruthenium and rhodium are representative of the group VII metals. Substantially all of the isotopes produced in the fission process are in the group having atomic numbers 30 to 64. Among these are isotopes of the non-metals bromine and iodine, but these isotopes have short half-lives and decay rapidly enough so that their disposal is not a pressing problem. The remaining elements in this group (other than the noble gases) have metallic properties and to these my invention is applicable. Of course, these isotopes should be in cationic form wherever they have amphoteric properties, e. g. arsenic, antimony, etc. For that purpose an acid solution of the fission products must be used. Other alternatives will be apparent to those skilled in the art.

Since many embodiments might be made of the present invention and. since many changes. might be made in the embodiment described, it is to be understood that the foregoing description is to be interpreted as illustrative only and not in a limiting sense.

I claim:

1. The method of disposing of radioactive cations in solution which comprises bringing said 9 solution into contact with a dried cation-exchange mineral consisting principally of montmorillonite to adsorb said radioactive cations on said mineral, thereafter heating said mineral to a temperature in the range 750 to 1100 0., and burying the heated clay.

2. The method of claim 1 wherein the radioactive cations consist principally of strontium.

3. The method of claim 1 wherein the radioactive cations consist principally of cerium.

4. The method of claim 1 wherein the radioactive cations consist principally of cesium.

5. The method of disposing of radioactive fission product cations which comprises bringing into contact a solution of said fission products and a dried cation-exchange mineral consisting principally of montmorillonite to adsorb said fission products on said mineral, thereafter heating said mineral to a temperature in the range 750 to 1100 C. and burying the heat-treated clay in a bed of natural clay.

6. The method of claim 5 wherein the mineral consists essentially of fullers earth.

7. The method of claim 5 wherein the mineral is bentonite.

8. The method of disposing of radioactive fission products which comprises bringing into contact a solution containing said fission products as cations and a dried cation-exchange mineral selected from the group consisting of montmorillonite, bentonite, and fullers earth to adsorb the fission products on said mineral, thereafter heating said mineral to a temperature in the range 750 to 1100 C. and burying the heated clay in a bed of natural clay.

9. The method of disposing of radioactive cations in solution which comprises bringing said solution into contact with a cation-exchange mineral consisting principally of montmorillonite to 10 adsorb said cations on said mineral and thereafter heating said mineral to a temperature in the range 750 to 1100 C.

10. The method of disposing of radioactive cations in solution which comprises bringing said solution into contact with a cation-exchange mineral selected from the group consisting of montmorillonite, bentonite and fullers earth to adsorb said cations on said mineral and thereafter heating said mineral to a temperature in the range 750 to 1100 C.

1 The method of disposing of radioactive fission products which comprises bringing into contact a solution containing said fission products as cations and a dried cation-exchange mineral consisting principally of montmorillonite to cause said fission products to exchange with said mineral and thereafter heating said mineral to a temperature in the range of 750 to 1100 C.

12. The method of disposing of radioactive fission products which comprises dissolving said fission products in an aqueous medium, bringing the solution into contact with a dried cationexchange mineral consisting principally of montmorillonite to cause said fission products to exchange With said mineral, thereafter heating said mineral to a temperature in the range 750 to 1100 C. and storing the heated clay in a container impervious to leaching by ground water.

WILLIAM S. GINELL.

REFERENCES CITED The following references are of record in the file of this patent:

U. S. Atomic Energy Commission Bulletin AECD-2802Radioactive Waste Disposal by John A. Ayres, Dec. '7, 1949, Technical Information Div., ORE, Oak Ridge, Tenn. 

9. THE METHOD OF DISPOSING OF RADIOACTIVE CATIONS IN SOLUTION WHICH COMPRISES BRINGING SAID SOLUTION INTO CONTACT WITH A CATION-EXCHANGE MINERAL CONSISTING PRINCIPALLY OF MONTMORILLONITE TO ADSORB SAID CATIONS ON SAID MINERAL AND THEREAFTER HEATING SAID MINERAL TO A TEMPERATURE IN THE RANGE 750* TO 1100* C. 